Sustainable development focuses on operational efficiency while promoting the minimization of environmental, social and economic impacts, demands a better utilization of our natural resources, especially the non-renewable ones. It is evident that the continued use of fossil fuels is no longer viable due to the depletion of global resources (Brennan and Owende, 2009). A promising solution to energy supply in the long-run is biofuels, and particularly those produced from microalgae. In addition to offering a solution that minimizes climate change by reducing CO2 emissions, which is ultimately achieved when the CO2 is metabolized by the microalgae in their growth and reproduction cycle. Such processes are more particularly generated through photosynthesis. Furthermore, microalgae are a rich source of valuable amino acids, proteins, pigments, vitamins, and antioxidants. Accordingly, microalgae offer key advantages over traditional feed stocks, such as fast photosynthetic growth rates and high lipid content, which can ultimately be converted into biofuels. The energy efficiency of microalgae had been reported to be 30 to 100 times greater than the energy efficiency of terrestrial plants. Also, as other biofuel sources (e.g. corn or sugar cane) the culture of microalgae does not necessarily compete with food supply.
Today, microalgae harvesting and dewatering remains a major obstacle to industrial-scale production of biofuels (Pienkos Darzins, 2009; Uduman et al, 2010). Although, several technologies for the separation of microalgae biomass are known, they still require large capital investments and/or large operational expenditures. The challenge of cost-efficient harvesting and dewatering of microalgae resides in their small size and low concentration in the culture medium. Existing harvesting and dewatering technologies such as centrifugal recovery and filtration require a relatively high amount of energy. Therefore, there is an interest in finding innovative methods for harvesting and dewatering microalgae with lower capital expenses (CAPEX) and operational expenses (OPEX).
Electrocoagulation and/or electro-floatation can be a competitive way to perform harvesting and primary dewatering of microalgae biomass as they both allow the destabilization of the suspended microalgae, followed by their aggregation into settleable and/or floatable flocs. The negative charge at the surface of microalgae creates repulsive forces between negatively charged particles, which cause them to remain suspended in solution. These repulsive forces can be weakened and cancelled by adding cations into the solution and thus lowering the charge of the microalgae. Cations of magnesium can be injected into the solution by electrolysis using a sacrificial anode made of a magnesium-based alloy. Simultaneously, gas bubbles produced at the electrodes lift the flocs towards the exit of the reactor. Moreover, many species of microalgae have a natural tendency to float since their cells contain relatively large quantities of low density lipids, which accelerates the floatation process.
Extraction of microalgae cells contents may be done using electrocoagulation, through the electric fields that helps permeating the cellular membrane. Lysis of the microalgae is driven by the oxidation process and by hydroxyl radicals that are a by-product of electrocoagulation. Electrolysis produces various oxidants, including hydrogen peroxide, ozone, chlorine, and chlorine dioxide. This method can be performed without the use of toxic solvents and chemicals. In addition to this, recent studies have demonstrated that electrocoagulation could also be used to discolor molecules when this is desired, and again without the use of toxic solvents and chemicals.
Microalgae typically range in size from 1 to 100 μm and they behave similarly to colloidal particles. As previously mentioned, freshwater and marine species of microalgae can be destabilized by making attractive forces between particles greater than the naturally occurring repulsive forces amongst them. Overall, the stability of particles in solution results from the sum of attractive van der Waals forces and of electrostatic forces responsible for the repulsion of particles, as well of residual forces originating from the steric effect of solvent molecules.
Coagulation can be achieved by chemical or electrochemical means. Chemical coagulation has been successfully used for decades, but it has a few shortcomings, which include risks for health and safety posed by the handling of hazardous chemicals and costs associated with the handling and treatment of the generated sludge that may contain relatively high levels of heavy metals. Moreover, traditional coagulation and flocculation techniques may use chemicals that are proven to be less effective in saline conditions.
Although the principles at works in electrocoagulation resembles that of traditional coagulation, there are some key differences between the two processes. Flocs generated by electrocoagulation differ from those generated by chemical coagulation because they tend to contain less bound water and to be more easily filterable. Moreover, harvesting and primary dewatering of microalgae using the magnesium-based alloy anodes enables one to maintain the heavy metal concentrations, particularly for Al and Fe, below the desired levels. Regarding the steps of extracting of microalgae, electrocoagulation and electro-floatation eliminates the use of organic solvents.
Electrocoagulation generates flocs from suspended solids, which ultimately aggregate together to settle or float in a liquid/solid separation tank. Currents of ions and charged particles created by the electric field in the reactor promote collisions amongst ions and particles of opposite signs that migrate in opposite directions, leading to an electrolysis induced coagulation.
Electrolysis reactions taking place at the surface of the electrodes are accompanied by generation of micro bubbles of hydrogen at the cathode(s) and of oxygen at the anode(s). These micro bubbles can further drive the upward movement of the microalgae flocs towards the exit of the reactor through floatation.
Applied electric current to a solution drives Faraday reactions at the interface between the electrodes and the treated solution, which leads to the establishment of chemical concentration gradients in the reactor. Depending on the design of the reactor and of the flow rate conditions in the reactor, a particular threshold of electro-kinetic energy can lead to the electrolysis of water, with the simultaneous development of pH gradients and with the transfer of electrolytic dissolution of the anode producing metal ions (Mg2+, etc.) or cations of the electrolyte from the anode to the cathode. The main electrolysis reactions taking place in the reactor include the following:
At the cathode, the main reaction is:4H2O+4e−→2H2+4OH−  (Equation 1)
The increase in hydroxyl ions can increase the precipitation of metal hydroxide. The pH of the cathode's region is basic. The following equations describe the chemical reactions at the anode:2H2O→O2+4H++4e−  (Equation 2)
If the anode is made of magnesium:Mg→Mg2++2e−  (Equation 3)
There is a growing interest for electrocoagulation to be used to discolor molecules when this is desired without the use of toxic solvents and chemicals. It does so rather economically by eliminating the trace amounts of chlorophyll that are present in the microalgae cells while performing the treatment. Currently, this is mainly done using activated carbon or discoloration agents, which are both expensive techniques.